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White Paper Energy-Efficiency Showdown: A Comparison Of Aeration Technologies On paper and in practice, aeration options and their corresponding efficiencies are calculated and compared. By Mark Gehring and John Lindam R esearch and experiences within biological treatment have led to greater ability among municipal water resource recovery facilities to review and upgrade equipment and subsystems to reduce energy consumption. In most cases, processes and equipment involving aeration are the major consumers of energy. It has been estimated that up to 60 percent of a plant's power consumption can be attributed to aerobic reactors in the activated sludge process of secondary treatment. In this article, factors pertaining to power requirement and oxygen transfer of aeration systems in biological treatment are discussed, with emphasis on the difference and effects on energy efficiency between fine-bubble aeration and mechanical surface aerators. In particular, oxidation ditches are discussed; with numerous installations in the U.S. and worldwide, they present a number of engineering challenges with respect to aeration and mixing. Oxygen Transfer And Efficiency In order to discuss aeration efficiency, the term "efficiency" must be clearly defined. It is generally acknowledged that oxygen transfer efficiency (OTE) plays an important role. Equation (EQ) 1: OTE = oxygen transferred/oxygen supplied Where: OTE = oxygen transfer efficiency (percent) Oxygen transferred = oxygen transferred from bubble to mixed liquor, kg (lb) Oxygen supplied = oxygen supplied aeration equipment, kg (lb) When OTE is measured under standardized conditions, as described by American Society of Civil Engineers (ASCE, 2007), the term Standard OTE (SOTE) is used. Comparing performance of different equipment for aeration must be done under standardized conditions to validate and normalize performance data. With regard to energy consumption, the common expression for efficiency is the standard aeration efficiency (SAE), which factors in pressure and efficiency of the mechanical equipment required to achieve a de facto oxygen transfer. The standard aeration efficiency is the amount of oxygen transferred per unit of energy consumed: 24 wateronline.com ■ EQ 2: SAE = SOTR/P Where: SAE = aeration efficiency, lb O2/horsepower-hour (kg O2/kWh) SOTR = oxygen transfer rate, lb/hr (kg/h) P = power required to transfer one mass unit of oxygen per unit time, HP (kW) It should be noted that the definition of power must always be stated clearly; herein, power is defined as the total input energy of any mechanical equipment required to achieve the rate of oxygen transfer for the biological process as stipulated by the European Committee of Standardization (2009) and ASCE (2007). Available Technologies There is a wide range of equipment suitable for delivering oxygen to aerobic reactors in biological treatment. Aeration devices, such as mechanical aerators and coarse and finebubble diffused aeration systems, cover a large portion of aeration equipment installed at water resource recovery facilities globally. The aeration equipment selection process should consider system capacity, operating range, and equipment reliability. Assessment of the aeration system performance should consider total system efficiency, evaluating individual aeration system component efficiencies along with the aeration control system. Table 1 shows a brief summary of estimated SAE values for common types of aeration equipment used for biological treatment. Aerator types include high- and low-speed surface aerators, submersed jet aerators, fine-bubble disc diffusers, and high density low flux (HDLF) fine-bubble diffusers. Table 1. Efficiency ranges for various types of aeration equipment (Tchobanoglous et al, 2003) Type SAE, kg O2/kWh SAE, lb O2/hp. h Low-speed surface aerators 1.5-2.1 2.5-3.4 High-speed surface aerators 1.1-1.4 1.8-2.3 Submersed jet aerators 0.9-1.4 1.6-2.3 Fine-bubble diffusers, discs 2-7 3-10 HDLF fine-bubble diffusers 3-8 5-13 Note that the SAE values in Table 1 should be used as reference; exact figures are equipment and application specific. Water Online The Magazine